1. Field of the Invention
The present invention is directed toward detection of concealed electronic markers that are commonly buried alongside pipes or cables and, in particular, to a digital implementation of a combined pipe and cable locator device to simultaneously search for buried markers with high accuracy and repeatability.
2. Discussion of Related Art
Utility conduits are often buried underground or concealed in walls and therefore are not readily accessible or identifiable. It is often necessary to locate these concealed utility conduits in order to repair and replace them. It is also important to know the location of utility conduits in order to avoid them while excavating an area. Examples of hidden utility conduits include pipelines for gas, sewage, or water and cables for telephone, television, or power.
There are various ways to locate concealed objects, for example, using line locators or marker locators. Line locators are appropriate when seeking electrically conductive objects, such as metallic pipelines and cables. Line locators may also be used for finding non-electrically conducting conduits when the conduit is marked with a conducting trace wire or trace tape buried along the conduit. The process of applying an AC signal to the conductor at an accessible point and detecting the resulting electromagnetic radiation is well known in the art. When an AC signal is applied, the conductor acts as an antenna radiating an electromagnetic field along its entire length that can be detected by a line locator.
In such an application, a line locator used above ground detects electromagnetic emissions from conductors underground. A disadvantage with relying solely on the line locator device is that it may fail to identify and distinguish among various types of utility conduits and conductors. Additionally, line locator devices cannot be used to locate non-conductive lines, such as, for example, gas lines, fiber optic lines and plastic water lines when those non-conductive lines are not marked with trace wires.
Conduits may also be marked with electronic markers, either at surface level or underground. Buried electronic markers may be used to locate and identify a number of concealed objects such as cables, pipes, access points, underground stock piles, survey points and septic tanks. Typically, marker locators locating passive, active, or smart markers generate an electromagnetic field that induces a response in the marker that can be monitored by a detector of the marker locator. Again, significant difficulty in marker type identification and depth determination may result, especially if multiple markers of differing types and depths are present.
A recent development is disclosed in U.S. Pat. No. 6,617,856 B1, which describes a DSP based marker locator that substantially reduces detection inaccuracies attributed to analog mixers, antenna saturation, and DC offsets common in many designs. A quadrature mixer and IIR adaptive filter are used to modify the integration (averaging) time of the system to enhance performance when the markers are deeply buried, and to allow a more responsive mode (shorter averaging times) when this is not the case.
It is desirable in such marker detection systems that the results be repeatable and accurate. Therefore, methods of detecting the maximum reflected signal strength of a marker at a certain depth are important. It is also important that marker locators determine with a high degree of accuracy the particular type of marker that is present. Therefore, there is a need to develop more highly accurate and reliable systems for detecting marker types and marker depths.